Months have now passed since the Islamist terrorist organization Boko Haram sparked international outrage by kidnapping at least 270 Nigerian school girls. In a rambling one hour video, the group’s supposed leader explained their actions to the world:

"I abducted your girls…I will sell them in the market, by Allah. I will sell them off and marry them off. There is a market for selling humans…Women are slaves. I want to reassure my Muslim brothers that Allah says slaves are permitted in Islam…I will marry off a woman at the age of 12. I will marry off a girl at the age of nine…”

Most of the media coverage has understandably focused on the apparent inability of the Nigerian government to rescue the girls, who were students at a Western-style boarding school. But the evil act also shined the light on two horrifying practices: child marriage and polygamy. For those of us who have lived our entire lives in societies that do not tolerate such things, the fact that they are still commonly practiced in some parts of the world is almost unimaginable.

In ancient times, including eras described in the Bible, both polygamy and child marriage were widely accepted. The practices were largely influenced by the shorter human lifespan (depending on one’s location, life expectancy may have been between 20 and 40 years), as well as the high incidence of women dying during childbirth. However, as the centuries passed medical technology and better nutrition extended the human life span and made childbirth much safer. And so the overwhelming majority of societies (particularly those influenced by Judaism and Christianity) outlawed marriage before puberty and marriage to more than one wife.

Such practices remain widespread, however, in some Muslim countries as well as the Sahel region of Africa (the semi-arid strip just below the Sahara), where experts estimate half of all women live in polygamous households. Furthermore, according to the relief organization UNICEF, many African countries still have shockingly high rates of child marriage. In Niger, 75 percent of girls are married before the age of 18. In Chad, it’s 72 percent and in Mali 71 percent, while well over half of girls marry as children in the Central African Republic and Mozambique. UNICEF estimates that at least 70,000 child brides die each year due to childbearing complications.

Nor are such practices necessarily fading away. This year, Kenyan President Uhuru Kenyatta signed the Marriage Act of 2104, which stated, “Marriage is the voluntary union of a man and a woman whether in a monogamous or polygamous union registered under the Act.” The legislature removed a clause from the bill which would have required that the first wife approve of any subsequent wives her husband took.

Although the debate in the United States is currently focused on whether the institution of marriage properly includes homosexual relationships, the practices of polygamy and child marriage in other parts of the world highlight that a society’s definition of marriage forms the foundation for its values. Societies that accept polygamy and child marriage dehumanize women and children. Societies with “flexible” definitions of marriage also have very malleable definitions of “right” and “wrong.” As Jillian Keenan argued last year in Slate, “The definition of marriage is plastic. Just like heterosexual marriage is no better or worse than homosexual marriage, marriage between two consenting adults is not inherently more or less “correct” than marriage among three (or four, or six) consenting adults.”

Some critics have scoffed at the notion that redefining marriage will lead to polygamy in the West, noting that places like Kenya still carry strong legal penalties for homosexuality. Since polygamy and child marriage are “traditional” in some parts of Africa, they feel they would be justifiable under the same reasoning that we “traditional marriage” advocates use to defend our cause. But the wonderful thing about denying moral relativism is that I don’t have to pretend that all traditions are equally valid. I can embrace the (largely Western) tradition of marriage as the lifelong union of one man and one woman, while rejecting the traditions of foot binding, female genital mutilation and polygamy.

The reason I have spent so much of my time and energy fighting for traditional marriage is that I know it to be the best possible way to arrange society. For five thousand years of human civilization, it did not provide the only model for family structure, but it consistently provided the best environment to raise children as well as the most just and humane arrangement for women, which brings out the best in men. That may sound to some like cultural imperialism, but I believe if more societies defined marriage exclusively as the union of one man and one woman, then men, women and children would all be better off.